Microcomputers, or personal computers (PCs), were initially designed for data processing.
It has been suggested to insert PC-format electronic circuit boards serving as an interface or a terminal adapter to provide a connection between a telephone set or a workstation and a network such as an Integrated Service Digital Network (ISDN). Such electronic circuit boards are, for instance, described in the following documents:
"ISDN attendant console" by J. Jaggernauth et al, "IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS 1986", Conference Record, vol. 2, p. 1247-1249, PA1 "Subsets, terminals, and terminal adapters for the public ISDN" by D. Adolph, "ELECTRICAL COMMUNICATION", vol. 61, no. 1, 1987, p. 72-80, PA1 "Les installations terminales d'abonnes" by J. L. Lavoisard et al, "COMMUNICATION ET TRANSMISSION", vol. 9, no. 3, September 1987, p.35-50, PA1 U.S. Pat. No. 4,640,989, and PA1 an "ISDN data and voice terminal based on a personal computer", B. Newman "GLOBECOM '85-IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE" Conference Record, vol. 3, p. 1048-1052. PA1 first electronic circuit boards on which are fitted first circuits providing the PC's central processing unit, input-output, and memory functions, and which are interconnected via a first bus, and PA1 at least a second electronic circuit board specifically incorporating interface circuits for connecting a plurality of terminals to an IDSN through line means, PA1 means being provided for switching of data transmitted over said line means so as to constitute a DST integrated within the PC,
These electronic circuit boards do not provide any switching function. The same is ture of electronic circuit boards intended to line a PC to a digital telephone set, one of the above two elements being connected to a private branch exchange (PBX) as described in the article "The PC/PBX Connection", by K. Pederson et al, AT & T TECHNOLOGY, vol. 1 no. 1, 1986, p. 36-37.
There have recently appeared Digital Subscriber Terminals (DSTs) for ISDNs that connect to a public network through several T interfaces. These DSTs (also known as subscriber mains) are capable of locally linking up telephone or data terminals basically by ensuring a switching function. DSTs thus appear as Intercom or PABX (Private Automatic Branch eXchange) type private exchanges that ensure both speech and data switching. A description of a DST is given in the J. L. Lavoisard et al article cited above.
It has also been suggested to integrate a PABX function into a PC. For instance, European Patent Application No. EP 0 192 894 discloses a microcomputer in which is inserted an electronic circuit board having line interface circuits, a user interface circuit and a PABX controller circuit interconnected via a circuit board bus. The PABX controller is connected to the microcomputer bus.
With this arrangement, the telephone function makes heavy use of the microcomputer's resources. Here, the telephone function, especially as regards switching of line-transmitted data, is managed on a time-sharing basis by the microcomputer's central processing unit. There is consequently a risk of disruption from applications other than the telephone function and which still need to be improved, such as writing into incorrect memory zones and overuse of common units (processor and memories). Moreover, a common bus is used for both the telephone function and the other microcomputer applications. There is therefore a risk of the bus being taken up by other applications at the expense of the data flow required for the telephone function. Finally, the inclusion of circuits providing a telephone function within a single board considerably increases the likelihood of faults developing and is incompatible with good modularity practice.
These same drawbacks are found with the solutions proposed in documents DE 3 534 918 and JP 58-69 193 since in both cases the telephone switching function is also ensured by the microcomputer's central processing unit.